5.7 swap with 1.84 A1G2 prop size

[Dre859]

So I swapped a 4.3 automotive motor for a 5.7 mercruiser. I have a 1.84 ratio as of now and put a mirage+ 23p on it and hit 48mph with 3 people. If my tack was correct the rpm was at 6k WOT. The boat is a 1994 20' rinker captiva 209 openbow. Has anyone done this swap with that gear ratio and what pitch propeller was best? I thought I would see speeds around 55mph-60 with this engine

 

[alldodge]

Your tach is off, at 6K the motor wouls start coming apart

I would say you need a 1:50 drive, but get a hand held tach first

 

[Dre859]

Yea I figured it was not at 6k but it's close from the sound of it. On the back of the tach there is a 4@9o'clock 6@noon 8@3o'clock. It was @ 6 the last time out. I put it on 8. Not sure if those numbers represent number of cylinders or not

 

[Scott Danforth]

If it was reading 6000 while set for 6 cylinders, you were running 4500 RPM

 

[Faztbullet]

Need the 1.5 ratio upper

 

[QBhoy]

You’ll need more than a 23” probably. Or If you get a 1.47/1.5 likely a 19.

 

[highpowerdad]

since it seems like we now know he is hitting 4500 rpm wot with a 23 pitch and 48 mph, why would he need to change the gear ratio, it isn’t overrev... assuming it’s not an Mpi engine isn’t wot 4400-4800, and something like 225 hp? Shy of 50 mph sounds right. Now if it is a 260 mpi, then 5k and low 50s would be possible, right, but that would be a lower pitch prop, not a gear change... what am I missing??

btw, just plugged the numbers into the Navi-computer 4500rpm, 23 pitch 1.82 ratio, 10% slip is 48 mph...

 

[Dre859]

Motor is a 1994 5.7 mercruiser I was told 260hp. What is MPI

 

[Dre859]

Would a 1.50 ratio speed the boat up?

 

[alldodge]

Would a 1.50 ratio speed the boat up?

No, and you no longer need to change to 1.5 drive

First comment was based on 2 unknowns
Tach is off so don't know what your turning
If it was over reving, then need a different ratio, and your not so the drive isn't needed

 

[highpowerdad]

Are you trimming up to get speed? Or are you leaving the drive all the way down. With a light load, (15 gal gas 1 or 2 people) properly trimmed etc, clean hull etc, my limited experience would put you at about 51 mph, maybe 52...and 4600 rpm, you could go down a little in pitch, ask a prop expert which prop for speed...3blade, what cup, etc, but if you got your rpms up to 4900 or so, you might gain 1 mph additional...but check your rpm accuracy and do a run properly trimmed first...you may already be there...

 

[Dre859]

I had 3/4 tank of fuel around 20gal. Did trim up for max speed. 3 people in the boat. Engine cover weighs 100lbs, I would like to make a light weight cover that is strong enough to walk on. Any ideas?

 

[Scott Danforth]

Any ideas?

it would help us help you if we knew what you wanted to do.

a carbon fiber engine cover could weigh as little as 30# and support 400#. however it will cost you nearly $2000 to build it right.

lets back up. your 20 foot boat weighed about 3200# dry weight with the V6. you added about #150 going to the V8.

you are not sure if youre prop'ed right

top speed with the V6 was about 42-44, correct?

to get it to 55-60mph, you will need a whole lot more motor than a stock 350 that you didnt give us any specs on. since you dont know what MPI is (its Multi-Port fuel injection). help us help you. what serial number of your current motor, or at a minimum, year and is it a 2-barrel or 4-barrel?

so what is it you want to do?

if you want to go faster than the 48mph, you may be able to eek out another mph or so with your current set up by dialing in the prop.

if you want to go 7-12mph faster, you will need between 70 and 120 more hp at a minimum

 

[Faztbullet]

Would a 1.50 ratio speed the boat up?

Yes.....a 4500RPM turning a 21 pitch is right at 55MPH depending on slip figures...that hull was remodled as the 212 and with a 5.7 L it tested at 52MPH

 

[Scott Danforth]

Yes.....a 4500RPM turning a 21 pitch is right at 55MPH depending on slip figures...that hull was remodled as the 212 and with a 5.7 L it tested at 52MPH

remember, in 96, the motors came out with vortec heads. we still dont know what motor the OP has in the boat, we dont know year, which heads, which fuel system, etc. since his current motor is only spinning a 1.84:1 drive with a 23p at 4500, I doubt there is enough motor to spin a 21p prop with a 1.47:1 drive at the same RPM